This application claims priority of DE 198 59 806.8-21, filed Dec. 23, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a steering system provided for motor vehicles, having a steering handle which can be operated on the driver side, particularly a manual steering wheel, a steering adjusting assembly for the motor-driven steering adjustment of steered vehicle wheels, a desired steering angle value generator operated by the steering handle, and an actual steering angle value generator operated by the steered vehicle wheels. A control system operates the steering adjusting assembly as a function of a desired-actual value comparison of the steering angle. A manual force adjuster which, in correlation to the forces which occur between the steered vehicle wheels and the steering adjusting assembly, causes a reaction force on the steering handle as well as an adjusting stroke of the steering handle correlated with the steering angle changes of the steered vehicle wheels. An emergency system, which becomes automatically operative in the case of disturbances of the control system and assigned system parts, provides for the mechanical and/or hydraulic forced coupling of the steering handle and the steered vehicle wheels with one another.
Steering systems of the above-mentioned type are currently being developed. The xe2x80x9csteer-by-wirexe2x80x9d concept is implemented, i.e., in contrast to conventional steering systems, in the case of a normal operation, no forced coupling is provided between the steering handle or the manual steering wheel and the steered vehicle wheels; on the contrary, the steered vehicle wheels and the steering handle are operationally coupled with one another only by way of a control system. A manual force adjuster has the purpose of generating a steering resistance at the steering handle or at the manual steering wheel as a function of the driving condition. That is, the manual force adjuster causes a feedback of the steering forces effective at the steered vehicle wheels. Simultaneously, exterior effects can be avoided, such as shocks to the wheels when driving through potholes which, in conventional steering systems, result in knocks of the manual steering wheel.
In the event of a disturbance of the control system or of interacting system parts, a hydraulic or mechanical emergency system is provided which becomes automatically operative in the event of a disturbance. As a result, a hydraulic orxe2x80x94as in the case of previous steering systemsxe2x80x94a mechanical forced coupling exists between the steering handle and the steered vehicle wheels.
As a rule, the conventional type of steering system operates in the normal operation with a variable ratio between the adjusting stroke of the steering handle and the steering angle change of the steered vehicle wheels. In which case, it is, however, preferably provided that, when the steered vehicle wheels are in the straight-ahead position, the steering handle takes up a fixedly defined straight ahead position. The above-mentioned variable steering ratio generally differs more or less clearly from the ratio of the emergency system.
If, during a cornering, i.e., in positions of the steered vehicle wheels and of the steering handle steered out of the condition for straight-ahead driving, the emergency system is switched on, after steering back the steered vehicle wheels into their straight-ahead position, the steering handle will not take up its normal straight-ahead position, but will be adjusted more or less far toward the right or left with respect to this normal straight-ahead position.
Although this does not represent a dangerous condition in a technical sense, the above-mentioned effect can be irritating to the driver. This is particularly true during parking maneuvers, during which the driver comparatively often pays attention to the respective position of the steering handle. In normal road driving, a driver will, as a rule, pay almost no attention to the position of the steering handle and will align steering maneuvers only according to the respective driving route, but, in an unfavorable position, a manual steering wheel may impair the view of the instruments.
Similar problems may occur if the steering system is set up for a track-guided steering operation, in which the vehicle follows, for example, automatically a road-side marginal strip or the like. During such an operating phase, with track-guided operation, the steering handle can optionally be made to stand still in the normal straight-ahead position. Should the emergency system now be switched on, the steering handle, while the steered vehicle wheels are in the straight-ahead position, may under certain circumstances have a position which is extremely far away from the normal straight-ahead position.
An object of the present invention is to improve the emergency operation in the case of a steering system.
According to the invention, this object has been achieved in that, on the driver side, the emergency system can be centered, while an assignment caused by the switching-on of the emergency system is changed between the positions of the steering handle and of the steered vehicle wheels. Thereby, in the straight-ahead position of the steered vehicle wheels, the steering handle takes up a defined position.
The present invention is based on the general recognition of providing a manual and/or automatic adjustment for the emergency system in order to assign the normal straight-ahead position of the steering handle to the straight-ahead position of the vehicle wheels.
According to a currently preferred embodiment of the invention, a warning signal can appear when the emergency system is not centered in order to draw the driver""s attention to xe2x80x9cunusualxe2x80x9d positions of the steering handle.
According to a preferred modified embodiment of the invention, a computer-assisted centering mode, during which the forced coupling may optionally at least partially be separated, can be switched on. The computer accesses signals which reflect the positions of the steering handle and the steered vehicle wheels, particularly the signals of the desired steering angle value generator and the actual steering angle value generator, and automatically switches on the forced coupling when a relative position of the steering handle and the steered vehicle wheels has been reached which matches the centering and/or a relative movement between the steering handle and the steered vehicle wheels occurs which leads away from such a relative position.
In this centering mode, also in the event of a disturbance leading to the automatic switching-on of the emergency system, signals will still be regularly available for the positions of the steering handle and of the steered vehicle wheels. The reason is that, also in the event of a disturbance, the desired steering angle value generator and the actual steering angle value generator almost always remain operable. Correspondingly, a computer analyzing the above-mentioned signals can xe2x80x9crecognizexe2x80x9d whether or not, when the forced coupling is in effect, the steering handle is centered relative to the steered vehicle wheels or relative movements occurring between the steering handle and the steered vehicle wheels would result in an improvement or a deterioration of the centering.
It can optionally be provided that, when the centering mode is switched on, an automatic centering of the emergency system takes place. This will be possible if, despite the disturbance causing the switching-on of the emergency system, in addition to the sensor system for detecting the positions of the steering handle and of the steered vehicle wheels, assemblies, such as the steering adjusting assembly and/or the manual force adjuster are also available, by way of which, in a corresponding control by a computer, a relative adjustment becomes possible between the steering handle and the steered vehicle wheels.
For safety reasons, it can be expedient to permit the centering or to be able to switch on the centering mode only when the vehicle is stopped and/or at an extremely low vehicle speed. For the same reasons, only elements which do not self-lock should permit a separation of the forced coupling and/or a switching-on of the centering mode in the emergency operation.